This paper presents a review of the contributions on entropy generation of nanofluids and hybrid nanofluids in the different types of thermal systems for different boundary conditions and physical situations . The relevant paper s are classified into three categories: entropy generation in minichannel, entropy generation macrochannel and entropy generation in cavities. The viscous dissipative, streamwise , electromagnetic effects, as well as nanoparticles concentration, the temperature and the flow regime on entropy generation , were analyzed. The reviewed literature indicates that the implementation of nanofluids/hybrid nanofluids in microchannels, minichannels , and cavities may be an important alternative to the traditional thermal systems and an interesting topic of study.

Introduction

Micro/nano heat transfer has been an important issue for researchers in the last decades. The microdevices (devices with dimensions between 1 ?? and 1 mm) are successfully employed in a variety of fields such as electronic s, aerospace , telecommunication, biomedical and automotive industries to enhance heat transfer for heating and cooling . The heat transfer efficiency can be improved by several methods, such as the use of extended surfaces, the application of vibration to heat transfer surfaces, and the use of mini/microchannels. Also, heat transfer efficiency of the microdevices can also be improved by enhanced thermo -physical properties of the working fluid, especially the thermal conductivity and the specific heat [1]. The working fluids with the thermo – physical properties improved are called nanofluid/hybrid nanofluid . By combining nanofluid/hybrid nanofluid with the small channel’s dimensions of the thermal devices, it is to obtain devices that provide compactness, low thermal resistance and efficiency. The entropy generation analysis is a useful tool for performance optimization of the thermal systems. It is known as the addition of nanoparticles into the base fluid could be influence the total entropy generation. Thus , the use the nanofluids in the thermal systems decrease the temperature of the system and finally the heat transfer contribution to the total entropy generation rate decreases, while nanoparticles added into the base fluid increase the viscosity of the working fluid leads to increase pressure drop in the system. Many researchers have studied the entropy generation in order to find optimum conditions for different thermal systems and only a few reviews have been devoted to this topic.